PHP获取唯一标识UUID

封装函数如下：

function uuid($prefix = '') {
    $chars = md5(uniqid(mt_rand(), true));
    $uuid = substr($chars, 0, 8) . '-';
    $uuid .= substr($chars, 8, 4) . '-';
    $uuid .= substr($chars, 12, 4) . '-';
    $uuid .= substr($chars, 16, 4) . '-';
    $uuid .= substr($chars, 20, 12);
    return $prefix . $uuid;
}